This invention relates to a system and method for improving the spectrum line width and stability of a laser diode light source and, more specifically, to fiber optic gyroscopes including laser diodes.
Generally, conventional laser diode light sources are economical and readily available commercially. Conventional laser diode light sources are widely used in the telecommunication industry and are effective over a wide temperature range. For example, a conventional laser diode light source is effective over a temperature range of xe2x88x9230 to 75 degrees Celsius. However, conventional laser diode light sources are generally not applicable to fiber optic gyroscope applications due to their narrow spectrum linewidth and unstable light spectrum characteristics. These attributes cause sub-interferometer effects in the fiber optic gyroscope which result in a poor in-run bias stability and scale factor stability.
In the past it has been proposed to use special broadband lasers as light sources for fiber optical gyroscopes. Such lasers however are very expensive and, indeed, cost several magnitudes more than the types of laser diodes presently available for communications purposes. However, as noted above, while these diodes are available and are inexpensive, they have the characteristic of sharp or narrow bandwidths and thus have not proven usable for applications which require a wide bandwidth, such as fiber optical gyroscopes.
One proposal to mitigate the sub-interferometer effects is to use only the non-lasing operational region of the conventional laser diode as an operational light source for operating the fiber optic gyroscope. A drawback of this proposal is that the power level of the non-lasing operational region of the conventional laser diode is very low. Therefore, the angle random walk characteristic of the fiber optic gyroscope tends to be undesirable. Other conventional light sources, such as super luminescence diode (SLD) and pump light sources, require active temperature controls and are very expensive to operate.
Accordingly, there is a need for a system and method which use the lasing region of the conventional laser diode light source but which stabilize the spectrum linewidth of the light source. More specifically, there is a need for a fiber optical gyroscope which can be operated with laser light from a conventional laser diode.
The invention provides a system and method for improving spectrum linewidth and stability while using the lasing region of a conventional laser diode light source. The inventive system and method provide a specially varying signal for varying the drive current of the laser diode light source. By varying the drive current, we have discovered that the spectrum linewidth and stability of the conventional laser diode light source are significantly improved.
We refer to this specially varying signal as a dithering signal by analogy to the technique used in telecommunication, such as television broadcasting, representing the entire gray scale of a picture by picture elements with only one of two levels (xe2x80x9cwhitexe2x80x9d and xe2x80x9cblackxe2x80x9d) in which a multilevel input image signal is compared with a position dependent set of thresholds, and picture elements are set to xe2x80x9cwhitexe2x80x9d only where the image input signal exceeds the threshold.
In one embodiment of the invention, an analog dithering system is provided for generating an analog dithering signal. In another embodiment of the invention, a digital dithering system is provided for generating a digital dithering signal. The analog and digital dithering signals can be used in a plurality of applications where a variation of a light source""s drive current is warranted, such as in a fiber optic gyroscope.
We have found that when the dithering signal is provided to vary the laser diode drive current of a light source used to operate a fiber optic gyroscope, there is a significant improvement in the in-run bias stability and scale factor stability of the gyroscope. Further, the inventive system and method realize an ultra low cost light source that can operate in a wide temperature range, e.g., xe2x88x9230 to 75 degrees Celsius, and requires very low power consumption.